Method of forming a superconductor film

ABSTRACT

The present invention provides methods forming the superconductor of as-grown film of MgB 2  which is made with magnesium and boron ejected from a magnesium target and a boron target respectively each in simultaneously sputtering process. The as-grown film composed of a compound of magnesium and boron is a superconductor without annealed. The present invention can be applied to fabricate an integrated circuit of superconductor film, because the high temperature annealing process for the as-grown film of MgB 2  is not needed.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority Japanese Patent Application No. 2001-356802, filed Nov. 22, 2001 in Japan, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to method of forming a superconductor film composed of a compound of magnesium (Mg) and boron (B). The present invention relates to especially method of forming an as-grown film having a feature of a high critical temperature superconductor without annealed.

[0004] 2. Description of the Related Art

[0005] A MgB₂ compound features in its high transition temperature to a superconductor. The MgB₂ film can be integrated on a substrate to form an integrated circuit. Thus the MgB₂ compound is expected to apply to electronics devices such as a thin film device.

[0006] It is reported that the coherent length ξ (0) of MgB₂ is 54 Å, and the magnetic penetration depth is 140-180 nm. It is well known as method of making the superconductor of MgB₂ to form a bulk of MgB₂ by compression molding of powder of mono-crystal MgB₂ and anneal the bulk at a high temperature. As another method of forming the superconductor of MgB₂, it is known to form a thin film of a MgB₂ compound by using PLD (pulse laser deposition), and anneal the thin film of MgB₂ in the temperature of 600° C.-1200° C. to make the film superconductive. The PLD method is that which irradiates a laser beam on a boron target to evaporate boron atoms and deposit the boron atoms on a substrate, and the boron atoms on the substrate react with magnesium atoms in high temperature magnesium vapor to form the MgB₂ film on the substrate. Further the film is annealed at a high temperature so as to become a superconductor. Another method of forming the superconductor film of a MgB₂ compound by using the PLD is that which deposits magnesium and boron atoms on a substrate by laser deposition, and sets the substrate in magnesium vapor. The magnesium and boron on the substrate react in the magnesium vapor to form a thin film of the MgB₂ compound on the substrate. The film is annealed at a high temperature to have features of a superconductor.

[0007] All prior arts to fabricate the superconductor of the MgB₂ compound need high temperature annealing of 600° C.-1200° C. The necessity of the annealing process of the MgB₂ film to produce features of a superconductor makes difficult to fabricate a device such as a thin film integrated circuit on a substrate. Further it is not known to deposit an as-grown film of MgB₂ on a substrate which is made by simultaneous magnesium and boron sputtering, using a sputtering apparatus, and that the as-grown film of MgB₂ is a superconductor without annealed.

SAMMARY OF THE INVENTION

[0008] The present invention features method of forming a super conductor film of MgB₂ without annealing the as-grown film.

[0009] The present invention provides the method forming an as-grown film of a superconductor composed of the MgB₂ compound which is made with magnesium and boron ejected from a magnesium target and a boron target respectively in the simultaneously sputtering process. The as-grown film composed of the compound of magnesium and boron can be a superconductor without annealed. The as-grow film can be composed of a MgB₂ compound, or the film can be composed of one or plural of compounds of magnesium and boron different from the MgB₂ compound, a magnesium element and a boron element in addition to the MgB₂ compound.

[0010] As mentioned above, a superconductor film of MgB₂ made using the preset invention has features of a superconductor without annealed, and its critical temperature is about 6K-29K. The present invention can be applied to fabricate an integrated circuit of the superconductor film, because the high temperature annealing process is not needed to make the as-grown film of the MgB₂ compound have features of a superconductor.

[0011] The objects, advantages and features of the present invention will be more clearly understood by referencing the following detailed disclosure and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 shows a sputtering apparatus to implement the first embodiment of the present invention.

[0013]FIG. 2 shows an example 1 of resistance-temperature characteristics of a superconductor made by the present invention.

[0014]FIG. 3A shows temperature dependence of resistance-temperature characteristics of a superconductor made by the present invention, where the substrate temperature of 320° C.

[0015]FIG. 3B shows temperature dependence of resistance-temperature characteristics of a superconductor made by the present invention, where the substrate temperature of 350° C.

[0016]FIG. 3C shows temperature dependence of resistance-temperature characteristics of a superconductor made by the present invention, where the substrate temperature of 380° C.

[0017]FIG. 4 shows temperature dependence of critical temperature and residual specific resistance of a superconductor made by the present invention.

[0018]FIG. 5A shows resistance-temperature characteristics of a superconductor of example 2 made by the present invention using the sputtering apparatus shown FIG. 1.

[0019]FIG. 5B shows resistance-temperature characteristics of a superconductor of example 2 made by the present invention using the sputtering apparatus shown FIG. 1, where the axis of abscissas in FIG. 5B is enlarged and corresponds to the region of 0-50 on the axis of abscissas in FIG. 5A.

[0020]FIG. 6 shows a static-type sputtering apparatus for implementing the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021]FIG. 1 shows a plan view of an apparatus to implement the present invention, that is Carrousel-type sputtering apparatus for implementing the present invention. As shown in FIG. 1, the apparatus is composed of a reaction room 1, pipes 5 and 6 for supplying argon gas, an ion beam source 7, a buffering room 8, electric heaters 9 which are lamp heaters for heating substrates 2, vacuum pumps 10 and 11, and rotary pumps 12 and 13.

[0022] Substrates 2 on which the superconductor is formed are set vertically to the plan view on holders. The holders are a cylinder of 6 surfaces on which the substrates 2 are arranged. The cylinder rotates around an axis. The substrates 2 are for example Al₂O₃, MgO, SiO₂ etc.. A magnesium target 3 of purity of 99.9%, a boron target 4 of purity of 99.5%, an aluminum target 1 and a niobium target 16′ are set vertically to the plan view on the wall of the reaction room 1. Each target is faced to one of the substrates 2. The aluminum target 15 and the niobium target 16′ are not used for forming a MgB₂ film. The ion beam source 7 generates argon ions and the ions are irradiated to the substrates 2 to clean their surfaces.

[0023] The reaction room 1 is exhausted to vacuum by the vacuum pumps 10 and 11 and the rotary pumps 12 and 13. Further argon gas fills in the reaction room 1. High voltage is applied between the magnesium target 3 and the substrate 2, and between the boron target 4. DC voltage is applied between the magnesium target 3 and the substrate 2 so that the magnesium target 3 is positive voltage. AC voltage is applied between the boron target 4 and the substrate 2. The argon gas is discharged between the target and the substrate of each pair of the target and substrate. The gas discharges can be run simultaneously at each pair of the target and substrate, and each of the discharges can be controlled independently. During the discharge, the substrates 2 are heated by the electric heaters 9.

[0024] The discharge between the magnesium target 3 and the substrate 2 is DC discharge, applying DC positive voltage to the magnesium target 3. The discharge between the boron target 4 and the substrate 2 is high frequency discharge, applying high frequency alternating voltage between the boron target 4 and the substrate 2. The argon gas is discharged between the magnesium target 3 and the substrate 2, and between the boron target 3 and the substrate 2. The argon ions generated during the discharge result in ion bombardments to the magnesium target 3 and the boron target 4. As the result of the bombardments, atoms or ions of magnesium and atoms or ions of boron are ejected respectively from each target, when the argon atoms or ions impact the targets. The magnesium atoms or ions and the boron atoms or ions react each other in the reaction room 1 to produce compounds of magnesium and boron. The compound of magnesium and boron is deposited on the substrates 2 which rotate in a plane vertical to the plan view around an axis with high speed. Or the magnesium atoms or ions and the boron atoms or ions are deposited on the substrates 2, and they react on the substrates 2 to produce the compound of magnesium and boron. In the explanation as follows, the sputtering process in simultaneous ion bombardments to the magnesium target and the boron target is called simultaneous magnesium and boron sputtering.

[0025] The as-grown film formed by the above mentioned method is usually polycrystalline, which is composed of only MgB₂, or composed of the MgB₂ compound and one or more than one of a magnesium element, a boron element and compounds of magnesium and boron different from MgB₂. The film made by the present invention is usually composed of components mentioned above. However the present invention will be explained by a film composed of only a MgB₂ compound in the followings for explaining explicitly.

[0026] The substrates which are used in this embodiment of the present invention are Al₂O₃, MgO or SiO₂. The temperature of each substrate is 250° C.-400° C. in the as-grown film forming process. The purity of argon gas is 99.9999%. The reaction pressure is 2-5 mTorr. The rotation speed of the substrates is 50 rpm. The input power for the discharge between the boron target and the substrate is high frequency AC power of 800 W. The input power for the discharge between the magnesium target and the substrate is DC power of 300 W. The reaction time to form the film is 10-60 minute.

[0027]FIG. 2 shows resistance-temperature characteristics of the MgB₂ film made by the simultaneous magnesium and boron spattering using the Carrousel-type sputtering apparatus shown in FIG. 1. The substrate of the experiment is Al₂O₃ (1102), and as the film forming condition, the substrate temperature is 380° C., the input power for discharge the gas between the boron target and the substrate is AC power of 800W, the input power of the discharge between the magnesium target and the substrate is DC power of 300W, reaction pressure is 5 mTorr (argon gas pressure). Under the above mentioned condition, an as-grown film of MgB₂ is formed on the substrate, and the film has features of a superconductor without annealed. The resistance-temperature characteristics are shown in FIG. 2. The critical temperature Tc is about 28 K, which shows the film is superconductive at a high temperature.

[0028]FIGS. 3A, 3B and 3C show temperature dependence of the resistance temperature characteristics. The resistance-temperature characteristics are measured at various temperatures as parameter. FIGS. 3A, 3B and 3C show the resistance-temperature characteristics respectively at the substrate temperature of 320° C., 350° C. and 380° C. without annealing the as-grown film. The films are formed in the simultaneous magnesium and boron sputtering by the Carrousel-type sputtering apparatus.

[0029] The substrate (1102) is Al₂O₃. The input power for the discharge between the boron target and the substrate is AC power of 800W, and the input power for the discharge between the magnesium target and the substrate is DC power of 300W. The reaction pressure is 2 mTorr(in argon gas). The as-grown films formed the simultaneous magnesium and boron sputtering in the apparatus shown in FIG. 1 has features of a superconductor without annealed. RRR (Residual Resistance Ratio) is resistance ratio at 300K and 40K, which show the degree of film fineness as a superconductor. It is known that, greater the RRR and less the specific resistance, the film is nearer to metal.

[0030]FIG. 3A shows a case of the substrate temperature of 320° C. It is shown that the MgB₂ film of critical temperature Tc of 9K and RRR=0.99 can be formed without annealed. FIG. 3B shows a film formed on a substrate of which temperature is 350° C. It is shown that an as-grown film of MgB₂ of critical temperature Tc 19K and RRR 1.10 can be formed without annealed. FIG. 3C shows an as-grown film formed on a substrate of which is 380° C. It is shown that the MgB₂ film of critical temperature Tc of 25.6K and RRR=1.14 can be formed without annealed.

[0031]FIG. 4 shows dependence of the substrate temperature of critical temperature and residual resistance ratio of the film formed by the present invention. Relations of the critical temperature and the residual resistance ratio to the substrate temperature are measured at various substrate temperatures to form the as-grown film of MgB₂. FIG. 4 shows the results. The condition to form the film is same with the condition of FIGS. 3A, 3B and 3C. In FIG. 4, the circled black dots show the relation of substrate and critical temperature, the black dots not circled show the relation of residual resistance ratio to the substrate temperature. The critical temperature and the residual resistance ratio corresponding to the same substrate temperature are those of same MgB₂ film on the substrate. The films formed on the substrate of which temperature is lower than 300° C. is metal, and do not show features of a superconductor. In FIG. 4, the values of residual resistance ratio at 300° C. is higher than values at another substrate temperature. It is supposed to be effect of the high magnesium rate near the substrate. The films formed on a substrate of which temperature is between 320° C.-380° C., shows the high temperature superconductor feature of residual resistance ratio of 1.0-1.7.

[0032] As explained above, the as-grown film of MgB₂ formed on the rotating substrate facing to the magnesium target and the boron target in the simultaneous sputtering process, using Carrousel-type sputtering apparatus, has features of a high critical temperature superconductor without annealed. In the above mentioned embodiment of the present invention, the substrates are rotated with high speed, however, it has been found by the inventors of the present invention that an as-grown film of MgB₂ formed by the simultaneous sputtering process on a substrate immovable has features of a superconductor at a high temperature without annealed as well as the film formed on the rotating substrate with high speed in the simultaneous sputtering process.

[0033]FIGS. 5A and 5B show resistance-temperature characteristics of MgB₂ film which is made by an embodiment 2 of the present invention method. The axis of abscissas in FIG. 5B is enlarged and corresponds to the region of 0-50 on the axis of abscissas in FIG. 5A. The as-grown film of MgB₂ made by method of the embodiment 2 of the present invention is formed on a substrate immovable, which is different from the method of embodiment 1 rotating the substrates. The resistance-temperature characteristics in FIGS. 5A and 5B are those of MgB₂ film formed on a substrate 2 immovable, using Carrousel-type simultaneous sputtering apparatus shown in FIG. 1.

[0034] A substrate 2 is located so as to face to a middle between the magnesium target 3 and boron target 4 in the Carrousel-type sputtering apparatus, and fixed at the position. The substrate on which the MgB₂ film is formed is Al₂O₃ (1102). Conditions for forming an as-grown film of MgB₂ are the substrate temperature of 340° C. in the film forming process, the applying power for the discharge between the magnesium target and the substrate of AC power of 800 W, the applying power for the discharge between the magnesium and the substrate of DC power of 300W, the reaction pressure of 5.0 m Torr in argon gas and the reaction time of 20 minutes for forming the film. As shown in FIGS. 5A and 5B, the as-grown film of MgB₂ formed by the embodiment 2 of the present invention has features of the superconductor without annealed.

[0035] It is shown in FIGS. 5A and 5B that a film fabricated on the immobile substrate by simultaneous magnesium and boron sputtering has features of a superconductor. The film is fabricated by using Carrousel-type simultaneous sputtering apparatus in a mode which its substrate holder is not rotated. However the MgB₂ film of the present invention can be formed with a static-type simultaneous sputtering apparatus.

[0036]FIG. 6 shows an outline of the static-type simultaneous sputtering apparatus. As shown in FIG. 6, the static-type spattering apparatus is composed of a reaction room 1, pipes 5 and 6 for supplying argon gas, ion beam source 7, buffering room 8, electric heaters 9 which are lamp heaters for heating substrates 2, vacuum pumps 10 and 11, rotary pumps 12 and 13, a substrate transportation rod 14, an axis of substrate holder 15′ which rotates the substrate 2 in a horizontal plane, a shutter 16, 17 which controls a film thickness, blocking the flow of ejected magnesium or boron atoms or ions, and a heater 17′ which heats a substrate 2.

[0037] The operation of the apparatus shown in FIG. 6 is explained. Argon gas is entered into the reaction room 1. DC voltage is applied between the substrate 2 and the magnesium target 3 so as to the magnesium target is positive voltage. High frequency AC voltage is applied between the boron target 4 and the substrate 2. The argon gas is discharged between the magnesium target 4 and the substrate 2, applied the DC voltage, and the argon gas is also discharged between the boron and the substrate 2, applied the AC voltage. The discharge between the substrate 2 and the magnesium target 3, and the discharge between the substrate 2 and the boron 3 can be controlled independently in the simultaneous magnesium and boron sputtering. While the simultaneous magnesium and boron sputtering, the argon ions bombard the magnesium target 3 and the boron target 4 to eject magnesium atoms and boron atoms respectively from each target. The atoms or ions of the magnesium and boron react each other in the reaction room 1 to produce molecules of a MgB₂ compound. The MgB₂ molecules are deposited on the substrate and an as-grown film of MgB₂ on the substrate. Or the atoms or ins of the magnesium or the boron ejected respectively from each target are deposited on the substrate and the magnesium element and the boron element react to produce a as-grown film of MgB₂.The as-gren film has features of a superconductor without annealed.

[0038] MgB₂ film formed by the method explained above is usually polycrystalline. However a mono-crystalline or amorphous film of MgB₂ which is formed by the simultaneous magnesium and boron sputtering process has features of a superconductor without annealed. They are involved the scope of the present invention.

[0039] The many features and advantages of the present invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modification and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modification and equivalents falling within the scope of the invention may be included in the present invention. 

What is claimed is:
 1. Method of forming a superconductor film comprising: filling gas in a sealed container in which a magnesium target, a boron target and one or more substrates facing to the targets, applying voltage between the magnesium target and the substrate to discharge the gas and between the boron target and the substrate to discharge the gas, forming a film of a MgB₂ compound on the substrates, the MgB₂ compound generated from reaction of magnesium ejected in sputtering process at the magnesium target and boron ejected in sputtering process at the boron target; wherein the film is a superconductor without annealed.
 2. Method of forming a superconductor film in claim 1: wherein the film contains at least one of a compound of magnesium and boron different from the MgB₂ compound, a magnesium element, a boron element in addition to the MgB₂ compound.
 3. Method of forming a superconductor film in claim 1: wherein the film is polycrystalline, mono-crystalline or amorphous.
 4. Method of forming a superconductor film in claim 1: wherein DC voltage is applied between the magnesium target and the substrate to discharge the gas, and the AC voltage is applied between the boron target and the substrate to discharge the gas.
 5. Method of forming a superconductor film in claim 1: wherein the film is an as-grown film formed on the substrate of which temperature is set in a temperature of 300° C.-390° C.
 6. Method of forming a superconductor film in claim 5: wherein critical temperature of the film is 19K-29K.
 7. Method of forming a superconductor film in claim 1: wherein the substrate is rotating in a plane facing to the targets.
 8. Method of forming a superconductor film in claim 1: wherein the substrate is immovable in the film forming process.
 9. Method of forming a superconductor film in claim 2: wherein the film is polycrystalline, mono-crystalline or amorphous.
 10. Method of forming a superconductor film in claim2: wherein DC voltage is applied between the magnesium target and the substrate to discharge the gas, and the AC voltage is applied between the boron target and the substrate to discharge the gas.
 11. Method of forming a superconductor film in claim 2: wherein the film is an as-grown film formed on the substrate of which temperature is set in a temperature of 300° C.-390° C.
 12. Method of forming a superconductor film in claim 11: wherein critical temperature of the film is 19K-29K.
 13. Method of forming a superconductor film in claim 2: wherein the substrate is rotating in a plane facing to the targets.
 14. Method of forming a superconductor film in claim 2: wherein the substrate is immovable in the film forming proces. 